The subject invention relates to the preparation of zinc phosphate conversion crystal coatings, which may be in anhydrous or hydrous form. Such coatings can be deposited on ferrous metal, such as steel, or on non-ferrous metal surfaces, such as zinc or aluminum, to protect the metal surfaces from corrosion.
When high-temperature performance organic top coating systems (e.g. polyamide, polyphenylene sulfide, and polyquinoxalines) are applied directly to conventional crystalline zinc phosphate (Zn.Ph) hydrate conversion coat surface, high-temperature treatment of the topcoat to form a solid polymer film typically results in interfacial disbandment and separation due to dehydration of hydrous Zn.Ph crystals. This failure is associated with the formation of weak boundary layers and results in poor corrosion protection.
Poor corrosion protection also results from alkali-induced dissolution of coating layers caused by the attack of the hydroxyl ions generated by the cathodic reaction during the corrosion process.
To minimize corrosion, various methods have been devised. Early attempts at using phosphate coatings to produce a corrosion resistant surface include British Patent No. 731,882, published Jun. 15, 1955, employing phosphatizing solutions containing anions of orthophosphoric and nitrate acids, cations of zinc and nickel and/or cobalt, and lactic or glycollio acid, U.S. Pat. No. 3,597,283, issued Aug. 3, 1971 to Shee, enlisting solutions containing phosphate, zinc, nickel, cobalt or copper, magnesium, nitrite, and fluoride and/or chloride, and U.S. Pat. No. 3,850,700, issued Nov. 26, 1974 to Heller, using a phosphate solution including zinc oxide, phosphoric acid, nickelous oxide and nitric acid. Although Heller shows enhanced results when coated surfaces are baked at 300.degree. F. to 400.degree. F. for 2-10 minutes prior to the application of an electrophoretic paint, such treatment is insufficient to convert a hydrous zinc phosphate coating to its .alpha.-phase anhydrous form. Moreover, none of these references teaches or suggests the use of a polyelectrolyte, such as polyacrylic acid, polymethacrylic acid, polyitaconic acid and poly-L-glutamic acid.
Morrison, U.S. Pat. No. 3,837,928, issued Sep. 24, 1974, teaches a conversion coating. Essentially, Morrison uses a conventional phosphating liquid with the addition of a copolymer of an unsaturated carboxylic acid and a selected ethylenic monomer. However, no polymer corresponds to those used in the subject application. Brock, et al., U.S. Pat. No. 4,052,232, issued Oct. 4, 1977, teach that low molecular weight (leas than 50,000) soluble polymers comprising monomer moieties selected from acrylic acid, methacrylic acid, acrylamide and methacrylamide, when present in an acid metal phosphating solution modify the physical form of the sludge produced. However, there is no teaching to use a phosphating liquid containing cobalt or nickel. Moreover, only water soluble polymers having a molecular weight less than 50,000 were found effective in supplementing the phosphating process.
Steel is frequently galvanized--that is given a coating of zinc or an alloy of zinc--to provide protection against corrosion. For example, in the automotive industry, steel used for body panels, fasteners, and structural members in automobiles is often electroplated with zinc or a zinc alloy to provide corrosion resistance. Although such electrogalvanized steel ordinarily exhibits improved corrosion resistance relative to uncoated steel, electrogalvanized steel is generally subject attack by salt water. For example, a panel of electrogalvanized steel exposed to a spray of salt water will ordinarily develop a layer of "white rust," which represents deterioration of the zinc layer. Typically, after continued exposure to the salt-water spray, "red rust" will be observed, indicating that the zinc layer had deteriorated to such an extent that the underlying steel became exposed to the salt water. Electrogalvanized steel suffers the further disadvantage that paints, lacquers, and other conventional automotive finishes tend to adhere poorly to the zinc or zinc alloy surface.
It is an object of the present invention to overcome the drawbacks of known systems by providing zinc phosphate conversion coatings that inhibit oxygen reduction reactions and minimize alkali dissolution.